1. The excretory system (Malpighian tubules and hindgut) of the stick insect is described, in extension of the earlier description by de Sinéty.
2. Analyses have been made of the inorganic components of serum (haemolymph heat-coagulated and centrifuged), of urine (the fluid produced by the Malpighian tubules), of faeces, eggs and privet leaves.
3. In the adult female insect most of the calcium ingested leaves the body with the eggs.
4. The haemolymph has the characteristic mineral base pattern of the herbivorous insect; it appears to be supersaturated with magnesium phosphate. Its volume varies from 70 to 180 mm.3, with an average value of 130 mm.3, representing about 15% of the weight of the insect.
5. The urine has the usual high concentration of potassium; except for phosphate the other inorganic components are present in lower concentration than in the haemolymph. Urine is produced at a rate of about 6 mm.3/hr., implying complete turnover of the water of the haemolymph every 24 hr.
6. Except for a slight (20%) decrease in potassium concentration the composition of the haemolymph remains constant during fasting.
7. The regulatory powers of the excretory system have been tested by injecting NaCl and KCl into the haemolymph and are shown to be relatively feeble.